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Su-27/Su-30 Roll-Rate

KJ_Lesnick

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Is it my imagination or does the Su-27 and Su-30 have relatively low roll-rates compared to planes like the F-15?

KJ
 

flateric

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please follow the magic url http://www.secretprojects.co.uk/forum/index.php/topic,5679.0/highlight,red+flag+2008.html
 

KJ_Lesnick

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No offense. Maybe I'm missing something but I don't think I saw anything in the text pertaining to roll-rate...
 

Trident

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The Flanker has a pretty decent roll-rate per se (270°/s), but due to its dense engines being spaced widely apart it has a high moment of inertia so the rotation takes quite long to build up. Single engine aircraft (F-16, Mirage2000) are most impressive in this regard.
 

flateric

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I just suppose that maximum rol rate on all of 'em are much beyond max allowed G for pilot...
 

flateric

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KJ, Su-27SK Flight Manuals. Explore.
http://www.maxho.com/lj/Su27SK.djvu
http://www.maxho.com/lj/su27mh.djvu
 

KJ_Lesnick

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Trident

The Flanker has a pretty decent roll-rate per se (270°/s), but due to its dense engines being spaced widely apart it has a high moment of inertia so the rotation takes quite long to build up. Single engine aircraft (F-16, Mirage2000) are most impressive in this regard.

That doesn't sound all that high -- I thought the F/A-18's roll-rate was 720°/s
 

Trident

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IIRC the Hornet will do 290°/s, not all that much better. More to the point, the very snappy M2k does not roll any faster than the big Flanker - ultimately. What the roll rate figure in isolation doesn't tell you is that the Mirage reaches that value much sooner and that is what makes the difference when you watch both fly. It is more 'responsive', thanks to its higher angular acceleration.

The T-38 can *theoretically* achieve 720°/s (I think), but only if you keep on rolling long enough, which incidentally provides another good example that the final roll rate may not tell the whole story.
 

flateric

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well, I hardly can imagine myself turning with 270/sec rate...not talking of 720...
 

Trident

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Aerobatic planes such as the Extra 300 and Sukhoi Su-26 series achieve in excess of 360°/s, up to 400°/s. And unlike combat aircraft, they are responsive enough for their pilots to take advantage of the capability on a regular basis ;)
 

KJ_Lesnick

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When does the 270 dps (Su-30) or 290 dps for the F-18 take effect?

When the control surfaces reach full deflection? After a certain period of time? I'm confused here.
 

Trident

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After a certain time of maintaining full control surface deflection. Don't ask me how long exactly that works out to, but probably after about a full revolution - give or take depending on the specific aircraft design.
 

AeroJadeXG

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I can tell you what the physics say, but not necessarily get 'real' numbers, but it'll get you an idea.

At full deflection, the control surfaces are exerting a maximum roll moment (torque for the motorheads in here) that won't (significantly) change as the roll continues.
At the first instant of full deflection there is a moment but 0 roll rate. That moment remains constant, and the roll rate increases rapidly at first. As roll rate increases, there is a 'rotational drag' opposite to the roll moment which is caused by having to push surfaces like the wing and tail through the air at an angle of attack. Eventually the two forces are equal, and the plane reaches a steady-state roll rate.

The rate at which you accelerate into a roll has the following major contributions:
1) Moment of inertia about the nose-tail axis (mass of each infinitely small piece of the plane X the distance of each piece from centerline)
2) Roll drag (less important than MOI)
3)Maximum lift coefficient in both the + and - direction (only the smaller of the two matters, I imagine, or else the plane would cease to travel straight, it would corkscrew)
4) Distance from lateral aerodynamic center to centerline (imagine a point laterally, at which all of the lift can be said to be produced, and still create the correct moment)
5) Forward velocity. I imagine you know the lift equation, and in this case Lift X lateral aerodynamic center X2=roll force, and that depends on V^2
 

Ogami musashi

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Hello,

The Average roll rate (at the first roll) for a SU-27 is about 180°/Sec.
The 270°/sec is only a PR value never attained.

RUssian Knights said SU-35 (their prototypes) were 360°/Sec but i think they probably did exagerate the thing a lot.

AFAIK the Hornet 720° roll rate is in dive after 5 or 7 rolls, the average one roll is towards 270°/sec

Note that the initial roll rate is always higher, for example the falcon has a 392°/sec initial roll rate but drops (and is controlled by the FBWS) at 270°/sec.


Quite strangely new fighters are more in the 200°/sec values, but i bet they can attain them with heavy loads.
 

KJ_Lesnick

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Ogami musashi,

Note that the initial roll rate is always higher, for example the falcon has a 392°/sec initial roll rate but drops (and is controlled by the FBWS) at 270°/sec.

Why would the initial roll-rate be higher?


KJ
 

Ogami musashi

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The first initial roll rate is mainly due to the aero-mecanic effect of the rudders (their size and coefficients).
But the installed roll rate will depend on the wings lift,span,inertia and surface.

Thus a plane can have a fast roll response giving a high instantaneous roll rate and then having a stabilized roll rate much lower.

In the case of the F-16 the main parameter is FBW limitation for the pilot but there're still some drops as the roll rate gets stabilized.
 

flateric

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differential TVC is not needed and even could things worse in case of roll maneuver
 

Kryptid

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flateric said:
differential TVC is not needed and even could things worse in case of roll maneuver
Well, whether it's needed or not, the Su-37 and Su-35BM can indeed use TVC for roll control. The widely-spaced engines would actually add to the torque induced by differential pitching of the nozzles.
 

KJ_Lesnick

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Does the Su-27 or Su-30 in any way use differential flap (like the F-16), or leading-edge (like the F/A-18) movement as well as aileron and tailplane movement?

KJ Lesnick
 

Trident

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The control surface layout is generally very similar to the Falcon, with large inboard flaperons (but no ailerons outboard) and differential tail plane movement (tailerons) for roll control.
 

KJ_Lesnick

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Trident,

The control surface layout is generally very similar to the Falcon, with large inboard flaperons (but no ailerons outboard) and differential tail plane movement (tailerons) for roll control.

I'm confused by your statement, are you saying the F-16 has no ailerons, or just the Su-27?

KJ Lesnick
 

Trident

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Neither fighter has ailerons in the strictest sense. Both have large inboard trailing edge devices which perform the functions of both ailerons and flaps (hence "flaperons") while the outboard part of the trailing edge is fixed. As I said, the Falcon and Flanker are based on essentially the same approach in this regard.
 

KJ_Lesnick

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Trident,

Neither fighter has ailerons in the strictest sense. Both have large inboard trailing edge devices which perform the functions of both ailerons and flaps (hence "flaperons") while the outboard part of the trailing edge is fixed. As I said, the Falcon and Flanker are based on essentially the same approach in this regard.

Wow, you learn something new everyday


KJ
 

KJ_Lesnick

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I have three questions

1.) Which has a more profound affect on roll rate at low-alphas:
- Differential flap-movement?
- Differential spoiler-movement?

2.) Which has a more profound affect on roll-rate at high alphas:
- Differential flap-movement?
- Differential spoiler-movement?

3.) Would there have been any improvement in roll-rate on either the F-16, the Su-27 or Su-30 if it used ailerons as well as differential flap movement?


KJ Lesnick
 

cosmicpop

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Hi

I don't think the answers to your questions about spoilers are that simple.

Spoilers tend to be used on high-aspect radio wings such as airliners and combat aircraft with low wing-sweep/speed. For example, I think most jet airlines use spoilers wholly or partially for roll control, and combat aircraft such as the Tornado and F-111 during low sweep angles. The SEPECAT Jaguar also uses roll-control spoilers. The thing that most of these aircraft have in common is the use of full-span flaps for lift augmentation so leaving little or no space for ailerons.

As for your third question, I'm sure the designers of those aircraft chose two-third span ailerons/flaps for a number of reasons. Mechanical simplicity (and weight) being one. No need for high-lift double or triple-slotted flaps being another. Also - the most acrobatic aircraft in the world (such as the Su-26, Yak-55 and Extra 300) all use full-span ailerons so there must be something good about it! :)

James
 

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Check this video -

http://www.crazyaviation.com/movies/CA_SU-30.wmv
 

Avimimus

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Trident said:
The T-38 can *theoretically* achieve 720°/s (I think), but only if you keep on rolling long enough, which incidentally provides another good example that the final roll rate may not tell the whole story.

I think that it might spin even faster once one of the wings snaps off ;)
 

HeavyG

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cosmicpop said:
Hi

I don't think the answers to your questions about spoilers are that simple.

Spoilers tend to be used on high-aspect radio wings such as airliners and combat aircraft with low wing-sweep/speed. For example, I think most jet airlines use spoilers wholly or partially for roll control, and combat aircraft such as the Tornado and F-111 during low sweep angles. The SEPECAT Jaguar also uses roll-control spoilers. The thing that most of these aircraft have in common is the use of full-span flaps for lift augmentation so leaving little or no space for ailerons.

As for your third question, I'm sure the designers of those aircraft chose two-third span ailerons/flaps for a number of reasons. Mechanical simplicity (and weight) being one. No need for high-lift double or triple-slotted flaps being another. Also - the most acrobatic aircraft in the world (such as the Su-26, Yak-55 and Extra 300) all use full-span ailerons so there must be something good about it! :)

James
Most airliners use spoilers to kill lift on the wings after landing. In all the times I've flown in commercial jets, I've only seen spoilers deployed after touchdown. I think only a few business aircraft used it as well, the Mitsubishi MU-2 twin turboprop being one example.
 

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